1. Field of the Invention
The present invention relates to an image forming method and apparatus and an optical head.
2. Description of the Related Art
Electrophotographic image forming apparatus that forms a latent electrostatic image on a photosensitive member such as a photosensitive drum and develops the image by application of toner is known from conventional printers, copiers, and facsimile machines. Known types of electrophotographic printers include laser printers, light-emitting diode (LED) printers, and liquid crystal printers. An LED printer includes a print head, also referred to as an optical head or LED head, comprising one or more LED array chips providing a linear array of light-emitting diodes (LEDs), a rod lens array for focusing the light emitted by the LEDs, and driver circuitry for selectively driving the LEDs. This type of print head has a simple structure and an easily alignable optical system, as explained in, for example, Japanese Patent No. 2,766,431.
A problem that occurs in LED printers is that different LEDs may emit optical energy with different spatial distributions. These variations in energy emission distribution can cause printing irregularities.
An ideal energy emission distribution is shown in cross section in FIG. 1, in which the horizontal axis denotes position on the emitting surface of an LED array chip and the vertical axis denotes emitted optical energy or power. Curves L1 and L2 are two Gaussian distributions, L1 being comparatively peaked and L2 comparatively flat. Curve L3 is the sum of distributions L1 and L2. The ideal emission distribution of optical energy emitted from an LED is similar to curve L3, but in practice, the shape of the curve or the area under the curve, indicating the shape of the distribution or the total energy emitted, tends to vary from one LED to another. Possible sources of variation include, for example, optical variations in the lenses, variations in the optical output of the LEDs, variations in the electrical characteristics of the circuits that drive the LEDs, and variations due to the positions of the LEDs in the LED array chips.
A first example of the emission distribution Ds1 of an LED is shown in plan view in FIG. 2; a second example Ds2 is shown in FIG. 3. If these distributions Ds1, Ds2 belong to adjacent LEDs that are driven one after the other in two consecutive dot lines, the distributions may overlap as shown in FIG. 4 or fail to overlap as shown in as in FIG. 5, depending on which LED is driven first. Profiles of the combined distributions are shown in FIG. 6, in which the horizontal axis indicates position on line X-X in FIG. 4, and FIG. 7, in which the horizontal axis indicates position on line Y-Y in FIG. 5. The vertical axis in FIGS. 6 and 7 indicates optical power; ρ indicates a threshold level for image formation. The two distributions shown in FIGS. 4 and 5 reinforce each other in FIG. 6, but fail to do so in FIG. 7.
More specifically, in FIG. 6, area a of distribution Ds1 overlaps areas b and d of distribution Ds2, and area b of distribution Ds2 overlaps areas a and c of distribution Ds1, causing the optical power of the combined distribution to exceed the threshold value ρ in the region between the two distributions Ds1 and Ds2 so that the two LEDs form a joined pair of dots.
In FIG. 7, however, areas a fail to overlap, and they also fail to overlap areas c and d, so when the two distributions are combined, the optical power does not exceed the threshold value ρ between the two distributions Ds1 and Ds2, and the two LEDs form a pair of isolated dots.
This situation is troublesome when image data with multiple gray levels are printed by dithering. If the dither matrix leads to the driving of LEDs with energy emission distributions Ds1 and Ds2 in the pattern shown in FIG. 4, the printed image may appear darker than intended. Conversely, if the dither matrix leads to the driving of LEDs with energy emission distributions Ds1 and Ds2 in the pattern shown in FIG. 5, the printed image may appear lighter than intended.
Irregularities also occur when multiple gray levels are printed by controlling the amount of optical energy emitted by each LED. For example, the combined distribution in FIG. 7 may cause white flecks to appear in an area that should be all black.
The dashed lines in FIGS. 4 and 5 indicate the basic resolution or dot pitch of the LED printer. Ideally, the emission distribution of each LED should be indicated by concentric circles centered within the dotted cells, but in practice the distributions may be eccentric and elliptical, as shown. Even if the total optical output of each LED is adjusted to the same level, FIGS. 4 and 5 show that emission distribution variations can lead to uneven density in printed images and therefore to lowered printing quality.
It would be desirable if these printed density variations could be avoided despite differences in the emission distributions of different LEDs.